The market for the modern DIY diver is a bit different than the vintage market and the concave seat may be seen as being desireable. It does increase the flow rate slightly, but more importantly the three different heights allow a broader range of IP adjustment and allow the IP to be adjusted downward - something that is important with older Mk 10s that may have springs that have gotten stiffer with age.
I used Nitrile o-rings for years in deco applications and they are fine for nitrox use, despite popular opinion, but EPDM may sell better with the modern diver due the craze for Viton or EPDM for nitrox applications. Please avoid Viton as it wears very badly in dynamic applications. There were thrid party viton kits available for the Mk 5 and MK 10 a decade or so ago and they were problematic in the Mk 10, especially the dynamic o-rings.
A Mk 5 kit will cover the Mk 1, Mk 5, Mk 6, the breathing side of the Mk 7 and the Mk 8. A Mk 10 kit will cover the Mk 9 and Mk 10. I use Mk 5s, 8s, 9s and 10s, for stage and deco bottles and when I put a kit together, I just add the larger Mk 5 piston head o-ring and the smaller swivel cap o-ring to the kit so it can do double duty.
It simplies the logistics for the cost of 2 additional o-rings and Bryan might want to do the same.
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Scubapro has changed poppets several times in both the 109 and 156. The latest S-wing poppet works fine with the A148 rivet style seat from Trident. Even though the seat is slightly thicker than the SP verison, the orifice adjustment, spring pressure and lever height all work out fine and the regs breathe great with that seat. I think the dealer cost on them is $1.60 but is no doubt cheaper in bulk.
The latest 109 Duro poppet uses a standard flat rubber seat so that is a non issue as long as the user has the latest poppet.
For users with the older or original poppets for the 109 and 156 with the molded on seat, they will either have to get newer poppets through Scubapro, or Bryan will have to make a new version of the old poppets or reverse engineer poppets similar to the modern ones (check out what Aqualung did with the poppet on the Legend - it looks almost identical to the old metal and plastic Scubpro balanced adjustabe poppet but done in one piece of black plastic.) In any case, he will encounter customers with both adjustable and balanced adjustable second stages and poppets of both old and new styles in each.
The 108 second stage is easy - a flat generic seat and o-rings for the orifice, inlet swivel and LP hose.
What I would love to see are parts for the now unsupported Air 1 as it is a superbly performing reg with both left and right inlet options. They use D400 service kits which are still available, but the critical parts are the diaphragm and the metal clip that secures the diaphragm.
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